This error can occur when the site can't reach the file /store/pc/service/api/hash.aspx or gets an unexpected error on that page. If you visit that page on your store you should get a very brief message that just says Err. If you get anything else that means .NET is likely not set up correctly on the server.

If you get the basic Err message when you visit the page in your browser it may indicate that internal requests from scripts on your store that try to get information from a URL on the same domain are routed differently than requests from other servers. You may be able to solve this issue by adding the site to the localhost file on the server.

If those don't work we may need to know more about the way the server is set up and the files that are involved. The best way to get us this information is to put in a support ticket at https://www.productcart.com/support-request.asp which has a secure form once you log in.